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S. S. Harlan

Publications and source records attributed to S. S. Harlan.

18 recordsLinked to original sources
USGS2010

Paleomagnetic results from Tertiary volcanic strata and intrusions, Absaroka Volcanic Supergroup, Yellowstone National Park and vicinity: Contributions to the North American apparent polar wander path

We report paleomagnetic and rock magnetic data from volcanic, volcaniclastic, and intrusive rocks of the 55-44Ma Absaroka Volcanic Supergroup (AVS) exposed along the northeastern margin of Yellowstone National Park and adjacent areas. Demagnetization behavior and rock magnetic experiments indicate that the remanence in most samples is carried by low-Ti titanomagnetite, although high-coercivity phases are present in oxidized basalt flows. Paleomagnetic demagnetization and rock magnetic characteristics, the presence of normal and reverse polarity sites, consistency with previous results, and positive conglomerate tests suggest that the observed remanences are primary thermoremanent magnetizations of Eocene age (c. 50Ma). An in situ grand-mean for 22 individual site- or cooling-unit means from this study that yield acceptable data combined with published data from Independence volcano yields a declination of 347.6?? and inclination of 59.2?? (k=21.8, ?? 95 =6.8??) and a positive reversal test. Averaging 21 virtual geomagnetic poles (VGPs) that are well-grouped yields a mean at 137.1??E, 82.5??N (K=17.6, A 95 =7.8??), similar to results previously obtained from published studies from the AVS. Combining the VGPs from our study with published data yields a combined AVS pole at 146.3??E, 83.1??N (K=13.5, A 95 =6.2??, N=42 VGPs). Both poles are indistinguishable from c. 50Ma cratonic and synthetic reference poles for North America, and demonstrate the relative stability of this part of the Cordillera with respect to the craton. ?? 2009 Elsevier B.V.

Tectonophysics↗
USGS2010

40 Ar/ 39 Ar geochronology and geochemical reconnaissance of the Eocene Lowland Creek volcanic field, west-central Montana

We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in westcentral Montana. 40 Ar/ 39 Ar age determinations show that the LCVF was active from 52.9 to 48.6 Ma, with tuff-forming eruptions at 52.9 ?? 0.14 and 51.8 ?? 0.14 Ma. These dates span the age range of vigorous Eocene igneous activity in the Kamloops-Absaroka-Challis belt. The LCVF evolved upward from basal rhyolites (SiO 2 >71 wt%) to dacites and andesites (SiO 2 > 62 wt%). Compositional change parallels a transition from early explosive volcanism to late effusive activity. Four geochemical components can be detected in the rocks. A component with 206 Pb/ 204 Pb < 16.5 and epsilon;Nd near-15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with epsilon;Nd below-10 are dominated by a second component; hydrous rocks with 206 Pb/ 204 Pb > 18.3 and epsilon;Nd >-9 contain a third component; and an andesite with low Nd content and epsilon;Nd near-9 probably contains a fourth component. The first three components probably derive from the lower and middle crust, whereas the fourth is probably from the lithospheric mantle. ?? 2010 by The University of Chicago.

Journal of Geology↗
USGS2008

Paleomagnetism of Proterozoic mafic dikes from the Tobacco Root Mountains, southwest Montana

Paleomagnetic data from Proterozoic mafic dikes in southwestern Montana provides evidence for two distinct episodes of subparallel dike emplacement at ca. 1450 and 780 Ma. Published geochemical data from dikes in the southern Tobacco Root Mountains has identified three distinct compositional groups, termed groups A, B, and C. Geochronological data from the group A dikes yielded a Sm-Nd age of 1448 ?? 49 Ma. Emplacement of these dikes is thought to reflect mafic magmatism associated with extension accompanying development of the adjacent Mesoproterozoic Belt Basin. Paleomagnetic results from these dikes and a group C dike yield antipodal magnetizations with a group-mean direction of D = 225.0??, I = 61.8?? (k = 27.9, ??95 = 7.7??, N = 14 independent means/24 sites). The average paleomagnetic pole (8.7??N, 216.1??E, A95 = 10.3??) is considered to be primary on the basis of positive baked contact tests and similarity to poles of ca. 1.45-1.4 Ga from intrusions elsewhere in North America, but is discordant with respect to poles from age equivalent sedimentary rocks of the Meosoproterozoic Belt Supergroup. 40Ar/39Ar dates from geochemical group B dikes are consistent with published U-Pb dates that demonstrate dike emplacement at 780 Ma as part of the regional Gunbarrel magmatic event. Hornblende concentrates from the group B dikes yield 40Ar/39Ar apparent ages of 778-772 Ma, whereas biotite from a baked contact zone yielded a plateau date of 788 Ma. Paleomagnetic results from the group B dikes yield a mean direction of D = 301.5??, I = -17.1?? (k = 65.7, ??95 = 4.0??, N = 12 independent means/23 sites) with a paleomagnetic pole at 14.6??N, 127.0??E (A95 = 3.2??). The combination of geochronologic data, results of a baked contact test, and spatial agreement of the paleomagnetic poles with poles of similar age elsewhere in North America indicates that this is also a primary magnetization associated with dike emplacement. Paleomagnetic data from some of the Tobacco Root Mountains dikes provide evidence that they were partially to completely remagnetized during latest Cretaceous to early Tertiary time, perhaps due to thermal affects associated with emplacement of the Late Cretaceous Tobacco Root Batholith. The overall agreement of paleomagnetic poles from the Proterozoic dikes with those of age equivalent rocks elsewhere in North America and agreement of the secondary magnetization with expected directions for the latest Cretaceous/early Tertiary indicate that the rocks of the Tobacco Root Mountains have not experienced significant tilting or vertical axis rotation since the Mesoproterozoic. The new paleomagnetic poles from this study thus provide key data for refining Meso- and Neoproterozoic parts of the North American APW path. ?? 2008 Elsevier B.V. All rights reserved.

Precambrian Research↗
USGS2003

Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Wind River Range, Wyoming, USA

We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U-Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170 ?? 8 Ma (95% confidence). An 40Ar/39Ar amphibole date from the same sample yields a similar apparent age of about 2124 ?? 30 Ma (2??), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355??, Inc.=65??, k=145, ??95=5??) with a paleomagnetic pole at 84??N, 215??E (??m=6??, ??p=7??). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed separation of the Wyoming Craton and Laurentia prior to about 1.8 Ga. Correcting the quartz diorite pole for the possible effects of Laramide-age tilting of the Wind River Range, based on the attitude of nearby overlying Cambrian Flathead Sandstone (dip=20??, N20??E), gives a tilt corrected pole of 75??N, 58??E (??m=4??, ??p=6??), which is also discordant with respect to time-equivalent poles from the Superior Province. Reconstruction of the Superior and Wyoming Province using a rotation similar to that proposed by Roscoe and Card [Can. J. Earth Sci. 46(1993)2475] is problematic, but reconstruction of the Superior and Wyoming Provinces based on restoring them to their correct paleolatitude and orientation using a closest approach fit indicates that the two cratons could have been adjacent at about 2.17 Ga prior to rifting at about 2.15 Ga. The paleomagnetic data presented are consistent with the hypothesis that the Huronian and Snowy Pass Supergroups could have evolved as part of a single epicratonic sedimentary basin during the Early Proterozoic. ?? 2002 Elsevier Science B.V. All rights reserved.

Tectonophysics↗
USGS2003

Exploration and discovery in Yellowstone Lake: Results from high-resolution sonar imaging, seismic reflection profiling, and submersible studies

Discoveries from multi-beam sonar mapping and seismic reflection surveys of the northern, central, and West Thumb basins of Yellowstone Lake provide new insight into the extent of post-collapse volcanism and active hydrothermal processes occurring in a large lake environment above a large magma chamber. Yellowstone Lake has an irregular bottom covered with dozens of features directly related to hydrothermal, tectonic, volcanic, and sedimentary processes. Detailed bathymetric, seismic reflection, and magnetic evidence reveals that rhyolitic lava flows underlie much of Yellowstone Lake and exert fundamental control on lake bathymetry and localization of hydrothermal activity. Many previously unknown features have been identified and include over 250 hydrothermal vents, several very large (>500 m diameter) hydrothermal explosion craters, many small hydrothermal vent craters (∼1–200 m diameter), domed lacustrine sediments related to hydrothermal activity, elongate fissures cutting post-glacial sediments, siliceous hydrothermal spire structures, sublacustrine landslide deposits, submerged former shorelines, and a recently active graben. Sampling and observations with a submersible remotely operated vehicle confirm and extend our understanding of the identified features. Faults, fissures, hydrothermally inflated domal structures, hydrothermal explosion craters, and sublacustrine landslides constitute potentially significant geologic hazards. Toxic elements derived from hydrothermal processes also may significantly affect the Yellowstone ecosystem.

Wyoming↗
USGS2003

Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions

Precise U-Pb baddeleyite dating of mafic igneous rocks provides evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. U-Pb baddeleyite analyses for eight intrusions from seven localities ranging from the northern Canadian Shield to northwestern Wyoming-southwestern Montana are statistically indistinguishable and yield a composite U-Pb concordia age for this event of 780.3 ?? 1.4 Ma (95% confidence level). This 780 Ma event is herein termed the Gunbarrel magmatic event. The mafic magmatism of the Gunbarrel event represents the largest mafic dike swarm yet identified along the Neoproterozoic margin of Laurentia. The origin of the mafic magmatism is not clear, but may be related to mantle-plume activity or upwelling asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by ???60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events provides unique time markers that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.

Geology↗
USGS2001

Paleomagnetic data bearing on style of Miocene deformation in the Lake Mead area, Southern Nevada

Paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late Miocene Hamblin-Cleopatra Volcano, which was structurally dismembered during the development of the Lake Mead Fault System (LMFS), provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the Basin and Range province. Sinistral separation along the fault system dissected the volcano into three major blocks. The eastern, Cleopatra Lobe of the volcano is structurally the most intact section of the volcano. Normal and reverse polarity data from paleomagnetic sites collected along traverses in the Cleopatra Lobe yield an in situ grand mean of Declination (D) = 339??, Inclination (I) = +54??, ??95 = 3.1??, k = 27.2, N = 81 sites. The rocks of the central core of the volcano yield an in situ grand mean of D = 3??, I = + 59??, ??95 = 6.8??, k = 42.5, N = 11 sites (six normal, five reverse polarity). Sites collected within the western Hamblin Lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of D = 168??, I = -58??, ??95 = 6.5??. k = 28.9, N = 18 sites. Interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area, considers the possibility of two different types of structural corrections. A stratigraphic tilt correction involves restoring flows to the horizontal using the present strike. This correction assumes no initial, possibly radial, dip of flows of the volcano and is considered invalid. A structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of D = 9??, I = +53??, ??95 = 3.1??, k = 27.2, and D = 58??, I = + 78??, ??95 = 6.8, k = 42.5, for the Cleopatra Lobe and the central intrusive core, respectively. The data from the Cleopatra Lobe are slightly discordant, in a clockwise sense, from expected middle- to late-Miocene field directions. The data from the volcano are not consistent with a proposed structural model of uniform, moderate magnitude, statistically significant, counter-clockwise vertical axis rotation of fault-bounded blocks during overall sinsitral displacement along the LMFS. We also analyzed dikes of the northernmost part of the Miocene Wilson Ridge hypabyssal igneous complex, strata of the Triassic Chinle Formation, and basalt flows of the Miocene West End Wash/Callville Mesa volcanic centers. Dikes in the Wilson Ridge pluton and the Triassic strata yield magnetizations with directions suggestive of statistically significant, clockwise, vertical-axis rotations consistent with local, large-magnitude shear of crustal fragments near some of the faults of the LMFS. Late Cenozoic deformation of the Hamblin-Cleopatra volcano area appears to have been non-uniform in scale and magnitude and no single structural model, involving strictly strike-slip faulting, can account for the observed paleomagnetic data. ?? 2001 Elsevier Science Ltd. All rights reserved.

Journal of Structural Geology↗
USGS1999

An ancient depleted mantle sample from a 42-Ma dike in Montana: Constraints on persistence of the lithosphere during Eocene Magmatism

Recent models for the Cenozoic tectonic evolution of the western margin of North America propose that delamination of ancient lithosphere accompanied asthenospheric upwelling, magmatism, and uplift subsequent to Laramide deformation. On the basis of the age of an alkaline dike in south-central Montana, thermometry of mantle xenoliths from the dike, and Sr, Nd, and Pb isotopic compositions of the dike and a xenocryst, we show that refractory lithosphere, derived from ancient, depleted mantle, remained in place under the Wyoming Craton as late as 42 Ma. The Haymond School Dike, a camptonite, yields a 40Ar/39Ar plateau date of 41.97 ?? 0.19 Ma (2??). Paleomagnetic data are consistent with this date and indicate intrusion during chron C19r. The dike has Sr, Nd, and Pb isotopic compositions similar to those of other Eocene alkaline rocks from central Montana. A clinopyroxene megacryst from the dike has ??42 = 17, and 87Sr/86Sr = 0.70288, indicating that it derives from ancient, depleted mantle isotopically distinct from the source of the host camptonite. Thermometry of xenoliths from the dike shows pyroxene populations that formed at 880?? and 1200??C. Combining thermometry with previous estimates of the regional Eocene geotherm inferred from xenoliths in kimberlites, and with the Al-in-orthopyroxene barometer, we infer that lithospheric mantle remained intact to depths of 110-150 km as late as 42 Ma. Eocene magmatism was not accompanied by complete removal of ancient lithosphere.

Journal of Geology↗
USGS1996

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5°N, 310.7°E ( K = 19.9, A 95 = 9.1°, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40 Ar/ 39 Ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9–1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40 Ar/ 39 Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40 Ar/ 39 Ar dating is applied.

Montana↗
USGS1996

The Eocene Big Timber stock, south-central Montana: Development of extensive compositional variation in an arc-related intrusion by side-wall crystallization and cumulate glomerocryst remixing

The Eocene Big Timber stock in the Crazy Mountains of south-central Montana is an elliptical, 8 by 13 km, compositionally and texturally diverse composite intrusion with a well-developed radial dike swarm. A sharp intrusive contact separates its two phases: the core of the intrusion is fine-grained quartz monzodiorite, and the volumetrically dominant remainder is composed of medium-grained diorite and gabbro. Differentiation-related major oxide variation within the stock is extensive and spatially nonsystematic. However, abundances of most trace elements were not strongly influenced by differentiation; late zircon and apatite fractionation caused moderate heavy and slight light rare earth element abundance depletions, respectively. Mineral compositions and assemblages indicate crystallization between ≈950 and 700 °C at a pressure of ≈0.8 kbar (3 km). Mixing models indicate that fractionation of varying amounts of plagioclase, orthopyroxene, clinopyroxene, magnesio-hastingsite, hornblende, biotite, titanite, apatite, and magnetite (the stock's principal constituents, with quartz and potassium feldspar) and remixing of these minerals and residual liquids controlled compositional evolution in the reservoir. Crystals apparently nucleated at the reservoir wall while residual silicate liquid was displaced inward and remixed. Some crystals were plucked from the solidification front, as indicated by glomerocrysts present throughout the stock, and also remixed with residual liquid. Solidification of the reservoir represented by the stock involved heat loss to enclosing wall rock, side-wall crystallization, and subsequent, variably effective, crystal-liquid remixing. This process is an important variant of conventionally invoked models pertaining to solidification of intrusions and explains extensive, relatively nonsystematic compositional variation. The genesis of compositional evolution in other intrusions characterized by extensive, spatially nonsystematic variation may result from the important process documented herein. Compositional and geologic relationships are consistent with magma genesis related to subduction and magmatic-arc processes inboard from the western edge of the early Cenozoic North American plate. Arc magmatism in south-central Montana during Eocene time is consistent with models pertaining to early Cenozoic southward sweep and westward retreat of magmatism. Magmatism represented by the Big Timber stock provides significant new support for steepening subduction, westward retreat of the subduction hinge line, and development of an asthenospheric mantle wedge that fueled renewed magmatism beneath the western edge of the North American continent following early Cenozoic shallow subduction.

Geological Society of America Bulletin↗
USGS1996

40Ar/39Ar geochronology and paleomagnetism of Independence volcano, Absaroka volcanic supergroup, Beartooth mountains, Montana

Independence volcano is a major volcanic complex in the lower part of the Absaroka Volcanic Supergroup (AVS) of Montana and Wyoming. Recently reported Rb–Sr mineral dates from the complex give apparent ages of 91 and 84 Ma, whereas field relationships and the physical and compositional similarity of the rocks with other dated parts of the AVS indicate an Early to Middle Eocene age for eruption and deposition. To resolve the conflict between age assignments based on stratigraphic correlations and Rb–Sr dates, we report new paleomagnetic data and 40 Ar/ 39 Ar dates for Independence volcano. Paleomagnetic data for the stock and an andesite plug that cuts the stock are well grouped, of reverse polarity, and yield a virtual geomagnetic pole that is essentially identical to Late Cretaceous and Tertiary reference poles. The reverse polarity indicates that the magnetization of these rocks is probably younger than the Cretaceous normal superchron, or less than about 83.5 Ma. Hornblende from a volcanic breccia near the base of the volcanic pile gives a 40 Ar/ 39 Ar age of 51.57 Ma, whereas biotites from a dacite sill and a granodiorite stock that forms the core of the volcano give dates that range from 49.96 to 48.50 Ma. These dates record the age of eruption and intrusion of these rocks and clearly show that the age of Independence volcano is Early to Middle Eocene, consistent with stratigraphic relations. We suggest that the Rb–Sr mineral dates from the Independence stock and related intrusions are unreliable.

Montana, Wyoming↗
USGS1990

Brunhes chron excursion/polarity episode recorded during the late pleistocene, Albuquerque Volcanoes, New Mexico, USA

All basaltic lava flows of the Albuquerque Volcanoes (lat.: 35.2°N, long.: 253.2°E), Albuquerque-Belen Basin, New Mexico, record a short excursion/polarity episode. K-Ar isotopic age determinations (weighted average: 155 ± 47 ka) and evaluation of soil profiles on flow surfaces suggest the late Pleistocene ( circa between 250 and 80 ka) as the time of extrusion. Results from 63 sites in a minimum of eight flows yield a mean direction of D = 101.1°, I = -36.1°, α 95 = 1.2° (α 1 95 = 0.7, α 2 95 = 1.2), k = 2219 ( N = 8 flows) and a corresponding virtual geomagnetic pole position (VGP) of 354.1°E, 20.2°S, dp = 0.8°, dm = 1.4° ( A 1 95 = 0.5, A 2 95 = 1.4), and VGP angular standard deviation (ASD) = 1.8°. The unusual magnetization in flows of the Albuquerque Volcanoes is carried by fine-grained [single-domain (SD) and/or pseudo-single-domain (PSD)] low-Ti magnetite and is apparently not a function of complex sub-solidus alteration of magnetic phases. Underlying baked soils contain a relatively low coercivity magnetization with directions comprising two groups. One is roughly antipodal to the lava remanence; the other intermediate between the lava remanence and present field direction. Because all flows yield statistically indistinguishable directions, a field directional path clearly cannot be defined. At the precision level of our age determinations, the Albuquerque feature may correlate with other short polarity episodes or excursions of late Pleistocene age (e.g., Blake or Jamaica). Though characteristically limited in directional morphology, well-dated polarity episodes (i.e. sub-chrons) and excursions recorded in volcanic rocks may provide information on the frequency of significant dynamo instabilities.

New Mexico↗